Night Games:

Aperature Priority Mode

ISO 3200 or HI 1

aperature as big as possible (smallest number)

Shutter speed faster than 1/250

White balance custom

Day games:

Shutter Priority mode

ISO auto or 200 to 800

white balance auto or sunny

Shutter speed 1/500 or higher

Bottom line is if your night football pictures are bad with these guide lines buy a 70-200 f2.8 VR, 80-200 f2.8 or 300 f2.8 lens or take pictures in the daytime.
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I would assume that you are shooting at the 200mm end, so the maximum aperture you have to work with is f/5.6, which is not particulary good at a high school football game. Not a disaster, but not good.

If the bleachers on the far side are clear, but the players are blurred, your shutter speed is too slow. Just setting the shutter speed to go faster is not necessarily a solution, as you get less light into the camera with each increase in shutter speed. You get darker images, to the point where there is nothing recorded at all. If you can do it, panning the shot will improve the image. Panning means you have the camera in motion, following the movement. You take the shot while moving the camera. In this case, the players would be (mostly) clear, while the far side bleachers would show the blur. It is something that has to be practiced to get even close to good at, but it isn’t that difficult. Cars are a good thing to practice on.. they go fast.

Turn off all that auto stuff… shoot the camera in manual mode. Set your ISo to 1600 to start, and use 3200 if you have to. Set the aperture as wide as possible, so f/5.6 and leave it there. Sports requires fast shutters, but give it a start at something like 1/125th and see what you get. Bump it up as much as possible, if you can get to 1/250th, you are good to go. If those images are too dark at that speed, back down one notch at a time until you find something acceptable between exposure and image quality.

This is why many people mention those f/2.8 lenses. A 70-200 non-VR (doesn’t help with sports anyway) from somebody besides Nikon can be had for as little as $700. The Nikon 70-200 f/2.8 VR just went up in price to $2,100. The advantage of any of them for sports is that you automatically get 2 more full stops of shutter speed over that f/5.6 lens you use now.
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Your problem is that you bought a camera that is too high above your skill level, and you are getting bogged down by the settings. I also would like to add that you are paying for a ton of features that you will never use. If you want to get better, then step down to a Nikon D40 or D3000, and use the preset setting mode for sports. Growing into the D300 is a bad idea. I came from 8 years of film SLR experience, work for a newspaper, got a personal SLR Nikon D40 that I periodically used for jobs, and I STILL took over a year and a half to feel like I outgrew Nikon’s most basic camera! (That camera by the way is Nikon’s best low-light camera, it has been on the sidelines with me at games that were being broadcast on ESPN). You are going to continue being a poor photographer until you start with something more of your skill level. Race car drivers don’t get F1 cars and Ferraris as their first car and get told "It’s ok, you will grow into it!" They get Civics and Miatas and go-carts until they understand enough of the basics of their craft to move up to a more professional car. I could give you Lance Armstrong’s bike, but you would never "grow into it" or win the Tour de France. Same thing with the camera. Use the D300 all you want, just understand that it is hurting you, rather than helping.

The bluriness is not caused by your focus, it is caused by your shutter speed. As a quick fix, put your camera on "A" mode, and turn the dial all the way so that the lowest aperture number is displayed. Take a test picture, and if the kids are still blurry, then turn up the ISO. If you can’t get the kids to stop being blurry by ISO 1600 or 3200, then you don’t have a good enough lens for the job. Time to go to the camera shop. The 70-200 f/2.8 IS costs about $2,200 and the 300 f/2.8 costs like $8,000. Happy shooting!
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You get the idea here. Your particular Nikon model should allow you to choose the shutter speed, while IT chooses the correct f-stop.

Now…..if you want those "frozen in time" shots of a moving subject, go with 500, 1/1000 ( a second )–or higher IF your camera supports such.

Don’t kick yourself hard if pictures come out "fuzzy" or "blurred"; remember—it’s the SECOND PERCEPTION you see in a picture that may show ART instead of a mistake!!!
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Blur background can be caused by you panning the camera. If you are moving the camera to follow the action, the background will be blurred, learn to like it.

Automatic focus is your best bet. Matrix because of the movement.

ISO sensitivity to ON because of the varying light, but limit it to 1600. Also experiment with various fixed ISO’s.

There are no sure bets. Taking many pix and varying the settings gives you the best chance. Try using Live view. It will give you instant feedback on your pictures.
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Anyway, relax. Leave your aperture as wide open as possible, as you’re clearly looking for speed, so just leave it at f/2.8 and shoot with the lowest ISO you possibly can. If the images are a bit soft (I don’t know what lens you’re really using), close the aperture to f/3.2 or so, but I can’t see that being an issue.

As for your focusing, leave your D300 on spot-focusing. If you’re shooting single people just leave it in the middle. At f/2.8, your subject will be nicely isolated from the crowd without being half in and half out of focus. In my experience with the D300, the autofocus isn’t particularly fast enough for action with the big rectangle.

I don’t know what you’re planning to do with your pictures if you really need to go beyond ISO 1600, find the high ISO noise reduction and set it to HIGH. It’ll soften your pictures a tiny bit, but that’s okay. It’s better than grain.

Anyway, just leave it on f/2.8, focus with the middle focus point and work out your shutter speed/ISO limitations from there. Hopefully it’ll be bright enough for you to shoot at a lower ISO. If you’re shooting things at a distance, you’ll probably have to go faster than 1/125 or 1/160 second to stop it blurring.
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Second, I use a D300 to shoot sports all the time. Sometimes I use the 18-200 mm VR lens, but not in dark venues.

High school fields are not usually lit well, so no matter what camera you use, lighting will always be a problem

Here is what sports photographers do. They get meter readings of the field lighting and then with that reading, shoot using the camera in the manual mode.

The settings I use when shooting pro basketball, I shoot at ISO 1600, with balance at 3140 Kelvin 1/250th at f/4 or 1/500th at f/2.8 The lighting on school fields are not that bright, but if you can shoot at 1/250th @ f/2.8, the blur will be reduced and if you shoot at the peak of action, the blur will be further reduced.
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Editorial and sports photographer